Obesity and obstructive sleep apnea (OSA) are independent risk factors for insulin resistance, systemic hypertension and cardiovascular disease. Both disorders are associated with increased sympathetic and inflammatory activity, and increased oxidative stress, presumed mediators of their shared clinical consequences. Due to conflicting results of previous intervention studies treating OSA patients with CPAP, we still do not know if treatment of obese OSA patients has a substantial benefit on these risk factors. In Aim 1, to determine the effects of obesity on the response to CPAP treatment in OSA patients, we will compare responses in daytime sleepiness, insulin resistance, and arterial blood pressure following CPAP treatment in obese and lean OSA patients, stratified by the amount of abdominal visceral adipose tissue, a fat depot associated with increased sympathetic activity, inflammation, and oxidative stress, and a more powerful predictor than BMI of adverse cardiovascular and metabolic outcomes. Aim 2 will determine the effect of CPAP treatment in these two groups on sympathetic activity, and inflammatory and oxidative stress biomarkers. Our overall hypothesis is that, adjusting for OSA severity and obtaining normative data from non-OSA subjects with comparable amounts of visceral adiposity (Aim 3), the two OSA groups will have comparable improvements in daytime sleepiness, but that the cardiovascular and metabolic improvements following CPAP therapy will be decreased in OSA patients with increased visceral adipose tissue. We anticipate that although there will be a greater absolute change in markers of sympathetic activity, inflammation and oxidative stress in obese compared to lean OSA patients following CPAP treatment, the levels will still be abnormally high in the obese patients resulting in the decreased improvements in insulin resistance, arterial blood pressure, and vascular health in obese versus lean OSA patients. Relevance: Obese patients are at increased risk of developing sleep apnea. Both obesity and sleep apnea are felt to increase the risk of diabetes, hypertension, and cardiovascular disease. The proposed research will begin to determine if treating obese patients with sleep apnea helps to reduce these risks. If the beneficial effects of CPAP treatment are reduced in obese compared to lean patients with sleep apnea, then treatment of sleep apnea in obese patients needs to be combined with effective management of their obesity.